I co-run a large FFRPG (running for over seven years now!) on second life called Midian City. It's a dark, cyberpunk RP sim with adult themes, so I guess if I could rate it it would be rated M.
We currently have a lot of openings for new players and are looking to get a lot of people in, especially the following types:
-Those who like to RP lab work
-Those who like to RP mech repair
-Those who like to run criminal gangs
-Those who like to RP out running bars or working in them
-Those who like to RP “hybrids” (think catgirls, only more than cats and not furry)
We do have a large populace and it's likely you'll work into deeper story through your play there. If you have Secondlife, or don't mind downloading it and starting up drop me an IM here or for a faster reply drop me a message in world (my name is “Ember Ronas” What a shocker) and I'll help you get set up.
Thanks for your time!